The cellular uptake and expulsion rates of length-fractionated single-walled carbon nanotubes (SWNT) from 130 to 660 nm in NIH-3T3 cells were measured via single particle tracking of their intrinsic photoluminescence. We develop a quantitative model to correlate endocytosis rate with nanoparticle geometry that accurately describes this data set and also literature results for Au nanoparticles. The model asserts that nanoparticles cluster on the cell membrane to form a size sufficient to generate a large enough enthalpic contribution via receptor ligand interactions to overcome the elastic energy and entropic barriers associated with vesicle formation. Interestingly, the endocytosis rate constant of SWNT (10(-3) min(-1)) is found to be nearly 1000 times that of Au nanoparticles (10(-6) min(-1)) but the recycling (exocytosis) rate constants are similar in magnitude (10(-4) to 10(-3) min(-1)) for poly(d,l-lactide-co-glycolide), SWNT, and Au nanoparticles across distinct cell lines. The total uptake of both SWNT and Au nanoparticles is maximal at a common radius of 25 nm when scaled using an effective capture dimension for membrane diffusion. The ability to understand and predict the cellular uptake of nanoparticles quantitatively should find utility in designing nanosystems with controlled toxicity, efficacy, and functionality.